(1) Field of the Invention
The present invention relates to the field of spinal systems for linking the vertebrae.
(2) Prior Art
There are two types of spinal links: osteosynthesis links, and dynamic links.
Spinal osteosynthesis links have the aim of setting the linked vertebrae in a specific configuration and of stabilizing them during bone fusion in order to permit fixed stabilization in the set position.
Dynamic links, by contrast, do not generate bone fusion but instead seek to reduce the stresses on the articular facets and on the intervertebral disks, by permitting certain movements, while at the same time realigning the vertebrae relative to one another, if necessary.
The present invention relates more particularly to a linking and dynamically stabilizing element for a spinal fixation system, designed to link at least two implantable connecting assemblies to one another, permitting certain possibilities of movement of one relative to the other.
Linking elements for dynamic stabilization are already known from the prior art.
European patent application EP669109 proposes a device for stabilizing adjacent thoracic vertebrae, comprising a band which is made of an elastic synthetic material and which has a round cross section, and at least two pedicle screws. The band can be inserted through the transverse hole in the head of the screws and fixed by a locking screw transversely with respect to the hole, that is to say in the direction of the screw axis.
The device additionally comprises a bearing element mounted on the band. This bearing element forms a pressure-resistant body in order to transfer pressure forces between the two screw heads. The cross section of the band fits in suitable holes of the bearing element and of the screw head in order to center the bearing element and the screw head relative to one another.
In addition, the band can be pre-stressed between two adjacent pedicle screws by a band extension which protrudes outside the pedicle screws in order to be able to mutually support the bearing element and the screw head on a bearing surface which is common to them and is arranged around the band.
A major disadvantage of this stabilizing device is that the band has to be slid into the hole formed in the head of the pedicle screws, and also through the bearing element(s). This first entails a difficulty, upon implantation, in passing the band through these elements, as the pedicle screws are already fixed to the vertebrae. Introduction of the band is not easy; but it means above all that the length of the bearing element has to be chosen before the band is put in place. It may well happen that the effective distance between the screws after tensioning of the band is not exactly the distance desired, and in this case the surgeon has no other choice than to dismantle the assembly made up of bearing element and band in order to introduce a bearing element of a different length. In fact, this system does not afford any freedom in terms of compression or distraction between the screws after the elements have all been put in place. Moreover, the viscoelastic material of the bearing element compresses when the band is tensioned, which further complicates the choice of length of the bearing element since this changes during tensioning of the band.
Another major disadvantage of the stabilizing device of the prior art is that the linking element takes up a considerable volume: of the order of 12.5 millimeters. In some circumstances, it is then difficult to avoid the linking element coming into contact with bones and this contact causing great pain.
Another major disadvantage of the stabilizing device of the prior art is that the linking element does not effect any torsional return in order to oppose pivoting movements of the vertebrae about the disks.
Another major disadvantage of the stabilizing device of the prior art is that the linking element cannot be curved in order to adapt it to the natural curvature of the lumbar spine.
Another major disadvantage of the device is the impossibility of performing surgical revision to extend the assembly, or of replacing the linking element by a rigid rod in order to obtain osteosynthesis without complete removal of the material beforehand.
Another disadvantage of the device is the impossibility of combining it with a conventional osteosynthesis system.